Penicillins, cephalosporins and cephamycins are the drugs most widely used in medical practice against bacteria-caused diseases. The biosynthesis of these important beta-lactum antibiotics remains in large measure an unsolved problem in biorganic chemistry. Norcardicins A and B, and the potent beta-lactamase inhibitors clavulanic acid and thienamycin are recently discovered beta-lactum-containing compounds with unusual structural features. A new theory of beta-lactam biogenesis is proposed and experiments are described to delineate the mechanistic steps that act in vivo to produce cephalosporin C, norcardicin A and clavulanic acid. Finally, biomimetic syntheses are outlined which seek to test the scope and validity of biogenetic proposals in workable laboratory reactions.